This invention relates to an electrochromic device using an electrochromic material which can alternately and stably exist in three different oxidation states and assumes different colors in the respective oxidation states. A typical example of the electrochromic material is Prussian blue.
In one type of conventional electrochromic display devices, a solid inorganic oxide film deposited on a transparent electrode serves as an electrochromic material. In the case of widely used tungsten trioxide, the electrochromic oxide film is colorless in its normal state and assumes a blue color when a negative potential is applied to the electrode in contact with the oxide film.
Studies have been continued on a number of organic and inorganic materials that exhibit electrochromic effects to seek out ones which are practical in electrochromic display devices and advantageous in some points, e.g. in the type of color change, in the ease of forming into a film and/or in the stability in the presence of an electrolyte. For example, Prussian blue has attracted interest mainly because it can easily be formed as a film by an electrodeposition process and, therefore, would be favorable for the manufacture of relatively large-sized electrochromic display devices. Prussian blue loses its characteristic blue color when electrochemically reduced to such an extent that the trivalent iron in its crystal lattice entirely turns into divalent iron. Besides, the blue color changes into light yellow when the entire iron in the crystal lattice of Prussian blue is oxidized to trivalent iron. However, such oxidation of a Prussian blue film in the presence of an electrolyte causes gradual decomposition of the Prussian blue film. Therefore, it is difficult to practically utilize the blue-to-yellow and reverse changes in the color of Prussian blue.